It's really pushing it, but I can somehow get the sisters being retarded, they're young, dumb and naive. But the mother also being retarded just makes no sense to me. She's the agency president ffs.
Even if the whole Manga is meant to be tongue in cheek, it's still pushing it.
It's like
The Peter principle on crack. The mother and sisters all seem to be sharing two brain cells that are both fighting for third place.
Although the two younger sisters secretly like the MC, it's still twisted, the youtuber is a Tsundere* and so didn't speak out (or did but with different implied meanings, happy her brother was kicked out of the house for his own wellbeing). The older of the two doesn't actually care about him, but rather the music he creates under his pseudonym of "V"... because despite being amazing at everything, our dear MC's creativity with names only extends far enough to rip off a cheap energy drink.
* Sigh... because of of course she is, nothing screams caring like an idiot with an emotionally tepid personality.